Cryogenic cooling of electronic circuit elements has been known and used in the manufacture of large scale computers. With a large scale computer and a large number of electronic assemblies to cool, cryogenic cooling may be a justifiable expense. However, while it is desirable to cool only a single electronic chip in a computer work station or a computer terminal, the cost and the equipment required to accomplish such cryogenic cooling is not justifiable. Because of the lack of adequate cooling, certain electronic chips may not be used in stand-alone terminals or in work stations because the electronic chip cannot be maintained at a cool enough temperature to function reliably or efficiently. The cooling of a small number of chips, all closely positioned to each other, may also be accomplished by employing the present invention in a larger size. Larger sizes may accommodate electronic assemblies having 2 to 16 chips. When the number of chips cooled increases, the heat load increases correspondingly. While it may be desirable, it may not be necessary to cool the chips to cryogenic levels. Presently, electronic devices typically operate in ranges from 0.degree. C. to 100.degree. C. The upper limit temperature of 100.degree. C. is not the desired operating temperature and to increase reliability of the chips the upper limit is reduced. Desired device operating temperatures from 0.degree. C. to 35.degree. C. are typical. Temperatures below 20.degree. C. are not attainable with unassisted air cooling, and temperatures between 20.degree. C. and 35.degree. C. usually can not be achieved with practical air cooled packaging. Accordingly, chilled water with its attendant plumbing requirements is typically used to limit the operating temperature, if the lower temperatures are to be maintained. Subambient temperatures are difficult to implement even with water, due to the condensate which readily forms on the cooling fixtures, and can damage electronic components.
As the power of computers increases and more of the processing capability of computers is packaged within stand-alone devices such as work stations and individual terminals or on a single module in a larger computer, it becomes necessary to adequately cool the electronic components of the terminal, work station or module to insure proper operation of the computer. In some instances, it is only necessary to cool a few of the electronic components, or possibly even as little as one electronic component, to substantially lowered temperatures.
When only a few or only one electronic component is to be cooled to lower temperatures than those attainable with conventional air or water cooling, the approach to cooling that element must, of necessity, depart from the conventional systems used in today's main frame computers with many large multi-chip modules, if for no other reason, than for cost and equipment requirements.
Significant amounts of research have been done on a refrigeration process and apparatus which utilizes thermoacoustic refrigeration. In a pulse tube, a form of thermoacoustic refrigeration, a gas is compressed and expanded in pulses and the affect of the alternating compression and expansion of the gas contained within the pulse tube apparatus is that heat flows or is transported from one end of the pulse tube at a lower temperature to the other end at a higher temperature. Publications which describe the structure and operation of pulse tube refrigeration apparatus include:
Development of A Practical Pulse Tube Refrigerator: Coaxial Designs and The Influence of Viscosity by R. N Richardson, Cryogenics 1988 Volume 28, August, pages 516-520; Valved Pulse Tube Refrigerator Development, by R. N. Richardson, Cryogenics 1989 Volume 29, August 1989, pages 850-853; Double Inlet Pulse Tube Refrigerator: An Important Improvement by Zhu Shaowei, et al., Cryogenics 1990 Volume 30, June 1990, pages 514-520; Pulse Tube Refrigeration Process by W. E. Gifford, et al., International Advances In Cryogenic Engineering 1965, pages 69-79; An Experimental Investigation of Pulse Tube Refrigeration Heat Transfer Rates, by Marc David, et al., CEC 1989; Development and Experimental Test of An Analytical Model of the Orifice Pulse Tube Refrigerator, by Peter Storch, et al., Advances In Cryogenic Engineering, Volume 33, pages 851-859; Influence of Gas Velocity On Surface Heat Pumping For The Orifice Pulse Tube Refrigerator, by J. M. Lee, CEC 1989, pages 1-8; Pulse Tube With Axial Curvature, by Yuan Zhou, et al., Advances In Cryogenic Engineering, Volume 33, pages 861-868; Pulse Tube Refrigerator Performance, by E. Tward, et al., CEC 1989, pages 1-6; A Comparison of Three Types of Pulse Tube Refrigerators: New Methods For Reaching 60K, by Ray Radevabaugh, Advances In Cryogenic Engineering, Volume 31, pages 779-789; and Development of A Single Stage Pulse Tube Refrigerator Capable of Reaching 49K, by Jingtao Liang, et al., Cryogenics 1990, pages 49-51.
Additional publications describing apparatus and processes related to the type of device disclosed herein includes: Thermoacoustic Engines by G. W. Swift, Journal of Acoustics Society of America, Vol. 84, No. 4, October 1988, pages 1145-1180; A Pistonless Stirling Engine--The Traveling Wave Heat Engine, by Peter H. Ceperley, Journal of Acoustics Society of America, Vol. 66, No. 5, November 1979; and Pulse Tube Refrigeration Progress, by W. E. Gifford, et al., International Advances in Cryogenic Engineering, Proceedings of the 1964 Cryogenic Engineering Conference, 1965, pages 69-79.
U.S. Pat. No. 3,237,421 to W. E. Gifford describes a pulse tube method of refrigeration and apparatus.
U.S. Pat. Nos. 4,114,380 and 4,355,517 to Peter H. Ceperley describe traveling wave heat engines.
U.S. Pat. No. 4,953,366 to Swift, et al., describes a serial acoustic cryo cooler.
U.S. Pat. No. 4,625,517 to Muller discloses a thermal acoustic device wherein rod like elements are contained in the chamber between the heat source and the heat sink.
U.S. Pat. No. 4,584,840 to Baumann discloses a cooling machine or heat pump of the thermal acoustic type.
U.S. Pat. No. 4,489,553 and U.S. Pat. No. 4,398,398 both to John C. Wheatly, et al., disclose a heat engine utilizing an acoustic drive to transfer heat from a heat source to a heat sink.
In one class of heat pumping machines, known as thermoacoustic heat pumps, acoustical or pressure pulse work is input to the machine, which causes heat to flow from a lower temperature heat source to a higher temperature heat sink. Thermoacoustic heat pumps are inherently more reliable than many other heat pumps since there are no moving parts at low temperature. Some devices of this type can have no moving parts at all. There are two mechanisms by which this heat pumping can be achieved. The first is surface heat pumping, which can occur in the presence of a standing pressure wave. The second is related to the conventional Stirling-cycle heat pumping process, and can occur in the presence of a traveling pressure wave.
In a typical embodiment of a stationary or standing pressure wave thermoacoustic machine, as described in U.S Pat. No. 4,489,553, a compressible fluid in a tube is cyclically compressed and expanded by a sinusoidal drive mechanism at one end of the tube, while the other end of the tube is closed. The frequency of pressure oscillations is that of a standing wave for the enclosure. A second, stationary thermodynamic medium, which can be uniformly spaced parallel plates, concentric cylinders, cylindrical or square rods, etc. is placed in the tube such that it lies between a velocity node and antinode (i.e. where velocity and pressure fluctuations are both non-zero). This second medium is commonly known as the stack. The reciprocal motion of the fluid relative to the stack makes possible the continual flow of heat from the end of the stack that is closer to a velocity antinode, where heat flows into the stack at one temperature, to the end that is closer to a velocity node, where heat is rejected from the stack at a higher temperature.
The following stack characteristics are known to be required for surface heat pumping: low impedance to fluid flow along the axis of the tube, low axial thermal conduction, a high surface area-to-volume ratio, and imperfect thermal contact with the fluid. The concept of imperfect thermal contact describes conditions where heat may be absorbed from or rejected to the fluid (i.e., the effective heat capacity of the stack media, considering the conduction penetration depth at the frequency of operation, is much larger than that of the fluid), and the thermal relaxation time for heat exchange between the fluid and stack is inversely related to the driving frequency. The dimensions and spacing between elements in the stack are determined by known optimization criteria. Heat flows into and out of a conventional standing wave thermoacoustic heat pump through the walls of the tube, to which the stack must be thermally anchored. A conventional method of facilitating the flow of heat from the tube walls to the ends of the stack is to make the ends of the stack from a second, high conductivity material, which is attached to the walls of the tube. This is in contrast to the low conductivity material that is required to meet the aforementioned criteria for low axial conduction and imperfect thermal contact with the fluid.
Of particular interest to the present invention is the co-axial arrangement, which is comprised of two concentric tubes that form a core volume and an annular volume. This co-axial arrangement has been applied to pulse tube thermoacoustic engines, in which heat pumping can be accomplished by both the standing and traveling wave mechanisms. Flow between the tubes is permitted at one end. In one version of the co-axial arrangement, the regenerator and driver are placed in the inner tube, and the pulse "tube" is the annular space between the inner and outer tubes. In another version, the driver and regenerator occupy the annular volume, while the core volume is the pulse tube.
The cryo cooler refrigerators described in the foregoing patents and publications are primarily research apparatus and the descriptions do not apply the apparatus to a use for which the devices may be advantageously adapted or do not teach how the pulse tube refrigeration unit is adapted for practical application.
Thermoacoustic refrigeration and particularly pulse tube type refrigeration apparatus appear, from the foregoing publications and patents, to have progressed to the point where the thermoacoustic refrigeration capability may be implemented and used in devices requiring reliable cost effective refrigeration in a relatively small volume.